A display device in which a liquid crystal panel or an organic EL (electro luminescent) panel is driven by thin film transistors (hereinafter referred to as TFTs) in so-called an active matrix manner has been used recently. In such a display device, there is provided on its glass substrate with the TFTs made of amorphous silicon (hereinafter referred to as a-Si) or polycrystalline silicon (hereinafter referred to as p-Si).
Among others, peripheral drivers which are integrated by employing the p-Si whose mobility is large enough for a high-speed operation have been widely used. However, a silicon device having a higher performance has been demanded, in order to integrate a system, such as an image processor or a timing controller, in which a higher performance is required.
This is because the performance of the transistor made of p-Si is insufficient to form the silicon device having a higher performance. Such insufficient performance attributes to (i) a localized level in the band gap caused by incomplete crystallinity, and/or (ii) the decrease in the mobility and/or the increase in an S-coefficient (subthreshold coefficient), each caused by defect around the crystal grain boundary and in the crystal.
In view of the circumstances, research has been made on a technology to form a silicon device having a higher performance (e.g. WO publication WO93-15589 (published on Aug. 5, 1993), J. P. Salerno, Single Crystal silicon AMLCDs, Conference Record of the 1994 International Display Research Conference(IDRC) p.39-44(1994). In the technology, preliminary formed is a device having thin film transistors made of a single-crystal silicon thin film, or the like. This device is adhered on an insulator substrate, so that a semiconductor device is formed.
The WO publication WO93-15589 discloses that a display of a display panel in an active matrix type liquid crystal device is prepared with the use of a semiconductor device in which preliminary-made single-crystal silicon thin film transistors are transferred onto a glass substrate via an adhesive agent.
Further, Japanese Unexamined Patent publication No. 11-24106/1999 (published on Jan. 29, 1999) discloses another technology for forming a high-performance semiconductor device. This publication discloses a substrate for liquid crystal panel-use in which two kinds of the semiconductor devices, each having a different property, are formed on a single substrate. According to the substrate for liquid crystal panel-use disclosed in the publication No. 11-24106/1999, polycrystalline silicon TFTs formed on one substrate are transferred to another substrate on which pixel regions with amorphous silicon TFTs have been formed.
However, the configuration, in which two kinds of semiconductor devices coexist on the insulator substrate, disclosed in the publication No. 11-24106/1999 gives rise to the following problems. One of the two kinds of semiconductor devices indicates devices (transferred device) such as transistors which are formed by transferring from another substrate. The other one indicates devices (deposited device which is a semiconductor device formed on the insulator substrate whose semiconductor film is formed by thin film deposition such as PECVD) such as transistors in which a semiconductor layer, formed on the insulator substrate by deposition or the like, are used as an active layer.
The semiconductor device includes various kinds of insulation films, such as gate insulation films, an interlayer insulator film, and so on. Namely, an insulation film of the transferred device and an insulation film of the deposited device are laminated in a region, in which the transferred device is formed, on the substrate on which the two kinds of semiconductor devices are arranged. As a result, such a region where the transferred device is formed has a thickness greater than that of a substrate having only a single kind of semiconductor device.
In part of the semiconductor devices, particularly in a substrate in which a semiconductor and a metal wiring layer are incorporated, contact holes are usually provided in an insulation layer, and the semiconductor and the metal wiring layer are electrically connected, via the contact holes, to metal wiring in a region other than the region where the semiconductor and the metal wiring layer are provided. However, the contact holes in the insulation layer are ordinarily formed by etching. As such, it is difficult to prevent an increase in a diameter of the contact hole, in cases where the insulation layer is thick, i.e., in cases where a hole depth of the contact hole is deep. This results in an increase in an aspect ratio of the contact hole, thus causing a disadvantage for high-density integration of circuits.